Add export-depends

**Describe the feature request**

I would like `.cabal` format to have `export-depends`. This field would act as `build-depends`, but these dependencies won't be passed to the compiler.

The motivation is to avoid false positives when using `reexported-modules`. The reexports are handled by Cabal itself, and as GHC doesn't see that usage, it may incorrectly report dependencies as unused. From GHC POV, it is correct, they are not true build dependencies.

Additionally it would be great if `reexport-modules` functionality reported unused `export-depends` to complement `-Wunused-packages` from GHC.

Start by reviewing the .cabal format's build-depends and reexported-modules handling, then trace how unused dependencies are reported alongside GHC's -Wunused-packages. Done means export-depends is supported without passing those packages to the compiler, and unused export-depends entries are reported.

Written by the indexing model from the issue text.